70

EYE, THE

For two different colours, red and blue say, at the same intensity,

1________1____

B *~clogl + d alogl + b'

In a series of papers 1 Ives and Kingsbury have shown the importance of visual diffusivity in the theory of the Flicker Photometer.

Many interesting phenomena related to the subject-matter of this and precedingparagraphs have been described by Shelford Bidwell,2 but a description would be outside our present scope.

IV. DIOPTRIC PROPERTIES

§ (20) ACCOMMODATION.—The power of the normal eye in repose is approximately 59 diopters. By the process of Accommodation the power can be varied at will so as to bring objects at various distances into sharp focus. This power is greatest in children and diminishes regularly with age. Its range is about 10 diopters at 20 to 25, about 2 diopters at 50, and is negligible above GO years of age. While these figures apply approximately to the conditions of daily use, the actual amount of accommodation called into play is not equal for all parts of the eye, the accommodation at the centre of the pupil being greater than near the periphery. This is due to the fact 3 that during accommodation the anterior surface of the crystalline lens increases in curvature near the centre but actually flattens near the periphery. Nature compensates for this by reducing the size of the pupil when the eye is accommodated for near objects, thereby cutting out the peripheral regions, which would otherwise spoil the definition. If the pupil is dilated by some drug which does not affect the range of accommodation,4 the deterioration of the definition for near objects is usually marked.

§ (21) THE PUPIL.—This is the name given to the circular aperture of the eye. It is usually concentric with the optic axis, i.e. with the line through the centres of curvature of the various refracting surfaces. The centring of the eye is never exact, though the departures from it are rarely important. The most common defect is that the centre of curvature of the cornea is not on the axis of the crystalline lens. The error may sometimes amount to a quarter millimetre. Occasionally the pupil is displaced outward from the axis. Relatively large defects of centring do not appear to detract from the acuity of vision.

The pupil diameter alters under various

1 Ives and Kingsbury, Phil. Mag., 1914, p. 708 ; Phil. Mag., 1916, p. 290; Ives, PMl. Mag., 1917, xxxiii. 360.

2 Proc. Roy. Soc., 1894, p. 132.

a Tscherning, Optique physiologique, pp. 160 and 168. 4 E.g. cocaine or homatropine..

influences. The mechanism by which this happens is complex and is not completely understood. As mentioned in the last paragraph it varies with the degree of accommodation and also with the intensity of the light. As regards the former effect, J. W. French 5 found it to be negligible until, for a normal eye, the distance of vision came within about 30 cm. Thereafter the contraction was rapid. In a particular experiment the pupil shrank from 6-5 mm. at 30 cm. to 3-5 mm. at 10 cm. As regards the second effect French found that the pupil area could be represented over a large range of brightness by the relation

Thus, in order to reduce the area of the pupil by half, the intensity would have to increase thirty-two times. The above equation is for the central area covered by the macula lutea. French also investigated the effect on the pupil of light falling on other zones of the retina. The most sensitive zone is the one just outside the macula, the index for this zone being — J-. On going further from the centre the effect diminishes, the peripheral zone being very insensitive. In French's words : " This portion of the retina requires all the light it can get, and the pupil opens out to nearly its maximum diameter and responds but little to variations of intensity."

It is usually stated that the two pupils always vary together, even if the intensity to which they are exposed is widely different. This view is undoubtedly based on considerable evidence under ordinary conditions of vision ; but French describes experiments in which one pupil remained of constant size under constant illumination, while the other varied over a wide range under varying illumination.

This independence may not be common to all eyes, but it is evidently unsafe to assume that in all circumstances the two pupils will be of equal diameter.

§ (22) THE LINE OP SIGHT : FIXATION. — When the observer " looks at " a particular object he is said to fix it. The line joining the front nodal point of the eye to the point of fixation is termed the line of sight. The image of the point of fixation is formed on the fovea centralis, but only a very minute area is fixed at a time. In fact, however close two points may be, provided they can be seen as separate points, it is also possible to say that one of them is being looked at rather than the other.

Contrary to what might be expected, the line of sight is not coincident with the optic axis. It is inclined inwards and downwards with respect to it by 5 to 7 degrees. Since it passes-

8 Trans. Opt. Soc., 1919, xx. 209.